1. Field of the Invention
The present invention relates to evaporation-adsorption refrigeration, and particularly to a metal organic framework modified with functionalized carbon nanotubes used as an adsorbent for solar adsorption refrigeration.
2. Description of the Related Art
More than half of the energy consumption, and consequently the carbon dioxide emissions, of modern buildings originate from air conditioning processes. These are traditionally based on electrically-driven, mechanical compression chillers and heat pumps or classical burner systems. This demand is expected to rise in the future due to increased living standards and global climate change. However, with alternative technologies, less exergetic (i.e., closer to equilibrium) forms of energy, even low-temperature waste heat from industrial processes, can be employed for both heating and cooling. Solar heat as driving energy is particularly interesting due to the high coincidence of cooling demands and solar irradiation. While multiple working principles for thermally driven heat pumps can be realized, the evaporation-adsorption method has proven most feasible for this purpose.
The evaporation-adsorption process renders cooling applications independent of electrical energy. If used for heating, the incorporation of environmental heat allows for considerable fuel savings. The coefficient of performance (i.e., the relation between useful and driving heat), power density, cost, and operating lifetime of the complete machine are governed by the sorption material and its figures of merit, i.e., porosity, water sorption capacity, hydrophilicity and hydrothermal stability. The achievable loading lift, and also the required desorption temperature, directly depends on the hydrophilicity of the material, i.e., the p/p0 value at which adsorption occurs (ideally at 0.05<p/p0<0.4 for realistic performance characteristics), and, thus, the shape of the adsorption isotherm. Apart from stability issues, this is the main reason why silica gel and zeolites are currently used in commercial systems, although several metal organic frameworks (MOFs) have already shown a higher water sorption capacity. Due to the outperforming microporosities of MOF materials, the development of long-term water stable MOF with sufficient hydrophilicity would obviously help to establish sorption cooling in general applications.
Despite their desirable properties, MOFs suffer from two major deficiencies relative to conventional adsorbents, such as silica gel, activated carbon and zeolites. Namely, a majority of these materials (MOFs) are known to disintegrate rapidly in an aqueous medium, while silica gel, activated carbon and zeolite adsorbents retain structural integrity inside water, and MOFs are characterized by considerably lower values of thermal conductivity relative to the conventional adsorbents. Amongst all reported MOFs, MIL-100 has been reported to possess the highest hydrothermal stability. However, its thermal conductivity is still below the minimum threshold required for an adsorbent material to be a considered viable option in an adsorption refrigeration system.
MIL-100 is a metal organic framework material, having the empirical formula 3D-{M3O(X)(H2O)2[btc]2.nH2O} (M=Cr, Fe, or Al; X=OH, F; btc=benzene-1,3,5-tricarboxylate (or trimesate)), and has received recent attention with respect to its catalytic, gas separation and gas storage properties. The porosity of MIL-100 originates from both 25 Å and 29 Å mesopores, which are accessible via 5.5 Å and 8.6 Å windows, respectively. As both MIL-100(Al) and MIL-100(Fe) form under quite harsh synthesis conditions (pH=0.6, T=210° C., and pH<1, T=150° C., respectively), a fundamental water stability can be anticipated. For environmental considerations, MIL-100(Fe) is far more desirable. It would obviously be desirable to be able to modify MIL-100(Fe) for efficient use as an adsorbent material in a solar adsorption refrigeration system.
Thus, a metal organic framework adsorbent for solar adsorption refrigeration and method of making the same solving the aforementioned problems is desired.